The present invention relates to an upper neutron protection device for a nuclear reactor assembly and more particularly for fast neutron reactors.
In such reactors, it is necessary to limit the activation of the secondary sodium passing through the heat exchangers and to reduce damage to the various primary components, particularly the reactor vessels. The standard method consists of confining the neutrons in the central area of the core, by placing on the periphery thereof assemblies only containing fertile material in order to absorb the neutrons and also by placing fertile material in the upper and lower parts of the fuel assemblies, on either side of the fissile material. The protection is completed by so-called "upper neutron protection" sleeves, positioned in the upper part of the assemblies.
In most existing systems, each assembly comprises in the upper part a head in the form of a sleeve and in the lower part a foot, which are disposed on either side of the assembly. The sleeve has a greater thickness than the assembly and the space provided in its central part makes it possible to circulate the cooling fluid, e.g. liquid sodium, from bottom to top through the assembly. However, it is known that the assemblies have to be replaced in the core and this takes place for each loading campaign of the latter, as a function of the irradiation undergone. Each spent assembly must be removed from the core and is then passed to a reprocessing installation, where the head, assembly and foot are separated from the needles or rods. As a result the sleeve is lost on each occasion, because it is fixed to the body of the assembly. Devices have also been proposed which have a removable sleeve, but the latter have a considerable wall thickness and often the fixing devices are complex, which increases the cost of the equipment and complicates disassembly.